1. Field of the Invention
The invention relates to a radar apparatus for measuring a distance to a detection object by using the well-known sensitivity time control (STC) technique, and more particularly to a technique of correcting, with raised accuracy, errors caused by the STC and dependent on the intensity of reflected waves.
2. Description of the Prior Art
FIG. 1 is a diagram showing how the received reflection signals are STC amplified in an STC-based radar system for determining the distance or range from the radar system to a target object. In such a system, the amplification factor (A.F.) of an STC amplifier is varied with the period of time from the emission of a transmission signal to the reception of the reflected transmission signal as shown in FIG. 1. FIG. 1 also shows pulse waveforms of received reflection signals Vi1 and Vi2 received at times t1 and t2 measured from the emission of respective transmission signals and STC-amplified versions Vo1 and Vo2 of the received reflection signals Vi1 and Vi2, respectively. In this case, as is well known in the art, the STC-amplified reflection signals Vo1 and Vo2 are distorted in the STC process. The distortion causes an error (denoted by xe2x80x9cxcex2xe2x80x9d) between the peak position Tip of a pre-STC-amplification received reflection signal Vi and the peak position Top of the STC-amplified received reflection signal Vo. This error xcex2 (=Vopxe2x88x92Vip) is hereinafter referred to as xe2x80x9cSTC-distortion errorxe2x80x9d. The shorter the distance or the signal transit time between the radar system and the target object is, the larger the STC-distortion error is as shown in FIG. 1. Thus, the STC-distortion error xcex2 depends on the signal transit time.
However, even if the distances to reflection objects are identical to each other, the intensities of refection signals from the reflection objects in a same range may vary depending on the reflectance of the reflection objects. FIG. 2 shows how the STC distortion of STC-amplified reflection signal is affected by the intensity of the received reflection signal. In FIG. 2, waveforms labeled xe2x80x9cLxe2x80x9d are for reference reflection signals of a predetermined level. FIG. 2A shows, for a smaller reflection signal, a pre-STC-amplification reflection signal Vsi and the STC-amplified version Vso of the signal Vsi. FIG. 2B shows, for a larger reflection signal, a pre-STC-amplification reflection signal VLi and the STC-amplified version VLo of the signal VLi. In FIG. 2, Vr is a reference voltage for determining the start timing and the end timing of each reflection signal. The error in the rising edges of the reference reflection signal L and each of the STC-amplified reflection signals Vso and VLo consists of a first error component D1 due to the intensity of the reflection signal and a second error component D2 due to the STC distortion. If the middle point Tc of the pulse width at the reference voltage Vr is calculated as the peak position Top of each STC-amplified reflection signal Vo, the peak position Top of each STC-amplified reflection signal Vo depends on the STC distortion and the intensity of the reflection signal. (The intensity of a reflection signal can be estimated by the pulse width measured by using the reference voltage Vr.) In other words, even if reflection objects are in an identical range, the peak positions of STC-amplified reflection signals from the reflection objects vary in response to the intensity of the STC-amplified reflection signals. Hereinafter, the error between the peak position of an STC-amplified reflection signal and the correct peak position (i.e., the time interval from which the true distance is calculated) is referred to as an xe2x80x9cerror due to received signal intensityxe2x80x9d or xe2x80x9cxcex1 errorxe2x80x9d. Since the STC distortion error component D2 is an error in the rising edge, the error of the middle time Tc is equivalent to the arithmetic average of STC distortion error components in the rising edge and the falling edge.
From the foregoing description, it is seen that the above-mentioned STC-distortion error xcex2 depends on not only the signal transit time but also the intensity (or the measured pulse width) of a reflection signal.
Therefore, what is needed is a method and a system for correcting an error due to waveform distortion caused by an STC process in distance measurement by using a correction value determined not only by the signal transit time but also by the intensity (or the measured pulse width) of a reflection signal.
Also, what is needed is an STC-based radar apparatus that corrects an error due to waveform distortion caused by an STC process in distance measurement by using a correction value determined not only by the signal transit time but also by the intensity (or the measured pulse width) of a reflection signal.
There have been proposed various error correction techniques for distance measuring systems.
For example, U.S. Pat. No. 5,805,527, which is a counterpart of Japanese Patent Application Publication No. 9-236661 (1997), discloses xe2x80x9cMethod and apparatus for measuring distancexe2x80x9d. Though the patent deals with an error caused by variation in the intensity of the reception signal, it does not mention the above-described STC-distortion error.
Japanese Patent Application Publication No. 7-71957 (1995) discloses a distance measuring apparatus. The distance measuring apparatus corrects an error due to the STC distortion. However, the error correction is done with a correction value determined only by the signal transit time or the distance between the apparatus and the reflection object.
Thus, the prior art has failed to meet the above-mention needs.
According to an aspect of the invention, the above-mentioned problems are solved by a method of measuring a distance to a reflection object in a radar apparatus that transmits a transmission signal and applies a sensitivity time control (referred to as xe2x80x9cSTCxe2x80x9d) process to a reflection signal from said reflection object to yield an STC-processed reflection signal. In the method, a quantity corresponding to the distance is obtained from a transmission time of the transmission signal and a detection time of the STC-processed reflection signal. The quantity is corrected considering an error which is caused by an STC distortion and depends on the intensity of the STC-processed reflection signal.
The correction of the quantity is achieved by correcting the quantity by using a first correction value associated with the intensity of the STC-processed reflection signal to provide a corrected quantity; and correcting the corrected quantity by using a second correction value associated with the corrected quantity and the intensity of the STC-processed reflection signal to correct the error regardless of the intensity of the STC-processed reflection signal.
The above-described method is preferably realized by a computer program. The computer program may be stored in a computer-readable storage media such as a flexible disc, a hard disc, a magneto-optical disc, CD-ROM, ROM, etc. and is loaded into a system RAM for execution if necessary. Alternatively, the computer program may be loaded into a system RAM via any network.
According to another aspect of the invention, there is provided a radar apparatus for measuring a distance to a reflection object. The apparatus transmits a transmission signal by using a laser diode for example and applies a sensitivity time control process to a received signal from the reflection object by using, for example an STC amplifier to provide an STC-processed signal. The radar apparatus includes a controller. The controller obtains a quantity corresponding to the distance from a transmission time of the transmission signal and a detection time of the STC-processed signal; and corrects the quantity considering an error which is caused by an STC distortion and depends on the intensity of the STC-processed reflection signal. The controller corrects the quantity in the above-described manner.
The radar apparatus may detect a pulse width of the STC-processed signal through a comparison with a reference voltage by using a comparator and a time measuring circuit for example. The detected pulse width is used as the intensity of the STC-processed signal in correcting the quantity and the corrected quantity.
Alternatively, in addition to the time measuring circuit, the radar apparatus may include a first comparator for detecting a wider pulse width of the STC-processed signal by using a lower reference voltage and a second comparator for detecting a narrower pulse width of the STC-processed signal by using a higher reference voltage higher than the lower reference voltage. In the event the narrower pulse width is obtained in addition to the wider pulse width, the controller corrects the quantity by using a first-class first correction value associated with the narrower pulse width to provide the corrected quantity; and corrects the corrected quantity by using a first-class second correction value associated with the corrected quantity and the narrower pulse width of the STC-processed signal. And, in the event only the wider pulse width is obtained, the controller corrects the quantity by using a second-class first correction value associated with the wider pulse width to provide the corrected quantity, and corrects the corrected quantity by using a second-class second correction value associated with the corrected quantity and the wider pulse width of the STC-processed signal.
The middle time of the pulse width of the STC-processed signal may be calculated as the detection time of the STC-processed signal in obtaining a quantity corresponding to the distance.